In many types of nuclear reactors, water is used as a coolant to transfer the energy from the reactor core for generating electricity. For example in a pressurized water reactor (PWR), water circulates through the reactor core and a primary loop system containing one or more reactor coolant pumps and one or more steam generators. In the steam generator, the heat from the primary coolant is transferred to a secondary loop of water which forms steam which then runs turbine electric generators. In a boiling water reactor (BWR), the water in the primary loop is under less pressure forming steam which is directly passed from the primary system to the turbine for generating electricity.
The piping of the reactor cooling system is usually made of stainless steel and to some extend Co alloys. The main surfaces inside the primary loops of a PWR and the steam generator tubes are made of Ni alloys such as Inconel™ or Incoloy 800. Under operational conditions of a nuclear reactor at temperatures of greater than 280° C., metal ions are leached out of the alloys of the piping and are dissolved and transported into the coolant. When passing the reactor core during operation, part of the metal ions are activated to form radioisotopes. During operation of the reactor these metal ions and radioisotopes are deposited as an oxide layer on metal surfaces inside the reactor cooling system.
Depending on the type of alloy used for a component or system, the oxide layers which are formed contain mixed iron oxides with divalent and trivalent iron as well as other metal oxide species including chromium(III) and nickel(II) spinels. Especially the oxide deposits formed on the metal surfaces of the steam generator tubes have a high chromium(III) or Ni(II) content which makes them very resistant and difficult to remove from the metal surfaces.
The need for the removal of these oxide layers arises from time to time due to the incorporation of radioactive matter that takes place during reactor operation: Over extended operating periods, the amount of the radioisotopes, such as Co-60, Co-58, Cr-51, Mn-54 etc., deposited on the inner surfaces of the reactor cooling system accumulates. This results in an increased surface activity or dose rate of the components of the reactor cooling system. The removal of this radioactive matter is often necessary to reduce the level of personnel radiation exposure before inspection, maintenance, repair and dismantling procedures are carried out on the cooling system, in accordance with the ALARA principle (As Low As Reasonably Achievable).
Many procedures are described to remove the oxide layers containing radioisotopes from metal surfaces of the cooling system in a nuclear reactor. A commercially successful method comprises the steps of treating the oxide layer with an oxidant such as permanganate in order to convert Cr(III) to Cr(VI), and subsequently dissolving the oxide layer under acidic conditions using a solution of an organic acid such as oxalic acid. The organic acid additionally serves to reduce a possible excess of oxidant from the preceding oxidation step, and to reduce the dissolved Cr(VI) to Cr(III) in the decontamination solution. An additional reducing agent can be added to remove the oxidant and convert Cr(VI) to Cr(III). The metal ions and activated radionuclides originating from the oxide layer and dissolved in the decontamination solution such as Fe(II), Fe(III), Ni(II), Co(II) and Cr(III) are then removed from the solution by passing them through an ion exchanger. After the decontamination step, the organic acid in the solution is decomposed by photocatalytic oxidation to form carbon dioxide and water.
In general, a plurality of treatment cycles comprising an oxidation step and an oxide layer removal or decontamination step are carried out in order to achieve a satisfactory reduction of activity on the metal surfaces. The reduction of surface activity and/or the dose reduction correlating to surface activity reduction is referred to as “decontamination factor”. The decontamination factor is calculated either by the surface activity in Bq/cm2 before decontamination treatment divided by the surface activity in Bq/cm2 after the decontamination treatment, or by the dose rate before decontamination treatment divided by the dose rate after decontamination treatment.
Moreover, either the entire reactor cooling system including auxiliary systems or portions thereof which may be separated from the remaining systems, for example by valves, can be subjected to a decontamination treatment, or individual components such as main coolant pumps can be placed in a separate containers and treated for removal of the oxide layer formed thereon.
EP 2 564 394 discloses a process for the decontamination of components or systems of a nuclear power station, for example of a pressurized water reactor (PWR). The method comprises several treatment cycles, wherein each cycle includes an oxidation step in which the oxide layer formed on the metal surface is treated with an aqueous solution containing an oxidant, and a subsequent decontamination step, in which the oxide layer is treated with an aqueous solution of an organic acid. At least one oxidation step is carried out in an acidic solution, and at least one oxidation step is carried out in an alkaline solution. The document claims that changing the pH value of the oxidant solution from acidic to alkaline or vice versa will increase the overall decontamination factor.
However, it has been found that the above described decontamination treatment still requires a plurality (>5) of treatment cycles being performed in order to achieve satisfactory results for dose reduction or activity removal, thus resulting in a high amount of radioactive waste produced therewith.